A new dirty tricks campaign to embarrass the Democratic frontrunner, John Kerry, backfired ignominiously yesterday when it emerged that a widely circulated photograph of a protest against the Vietnam war was a crude forgery.

The photograph, falsely credited to Associated Press, combined two separate images to make it appear as if Mr Kerry shared a stage at an anti-war rally in the early 1970s with the actress, Jane Fonda.

Ms Fonda is reviled by many Vietnam vets for her wartime visit to Hanoi, and the image was widely aired over the internet by a fringe group of Vietnam veterans who have pursued a vendetta against Mr Kerry for years.

In less than a week, the forgery travelled from a message board on a rightwing website to a Vietnam veterans' mailing list to mainstream organisations. Two British national newspapers - the Daily Mail and the Mail on Sunday - used the photograph in editions on Friday last week and at the weekend.

The revelation that the photograph was a fake follows the rumour that Mr Kerry had had an affair with a young trainee reporter.

That claim, which started on the rightwing Drudge report website, was largely ignored by American papers when it first surfaced, but was leapt on by some British newspapers.

On Monday the woman at the centre of the furore, Alexandra Polier, issued a statement in which she denied ever having had a relationship with Mr Kerry.

Inside the Kerry campaign, Democratic operatives said yesterday they were certain that the forged photograph would not be the last attempt to try to discredit their candidate's Vietnam war record.

"There are going to be a lot of dirty tricks in the campaign. It's like the story of the intern, which flew high as a kite before being shot down," said John Hurley, Mr Kerry's campaign adviser on veterans' issues.

Although it is possible to trace the Fonda-Kerry forgery through the internet, it was not clear yesterday who created the photograph. There was no direct evidence targeting the altered image to the Republican party.

However, the episode was seized on by Democrats as evidence of Republican trickery in an election season already notable for its low tactics.

The furore over the concocted photographs - and the smear that Mr Kerry had had an affair with Ms Polier - have also provoked a debate within the US media on its role in perpetuating campaign mud slinging.

Various Republican grandees and officials from President George Bush's re-election campaign have warned that operatives are mining Mr Kerry's 18-year voting record in the Senate, as well as his history as a Vietnam veteran turned war protester, in search of damaging material. Mr Kerry has formidable critics in a fringe section of war veterans who played a key role in the distribution of the forged image.

Although Mr Kerry is widely respected both for his courage in the Mekong delta and for his activism following his return from Vietnam, he has aroused the wrath of a small, but vocal minority.

Among his most devoted enemies over the years is a veteran from North Carolina. Ted Sampley, a former Green Beret, was bitterly opposed to Mr Kerry's leadership in re-establishing American relations with Vietnam, and openly accuses him of betraying missing US soldiers.

The election campaign has given new meaning to Mr Sampley's previously lonely campaign. On his website for veterans against Mr Kerry, he has portrayed the candidate in front of a Vietcong flag, and in a legitimate picture in a rally several rows behind Ms Fonda.

He also played a crucial role in circulating the forgery. "It was a dream come true that picture coming up at this point of time," he told the Guardian. "If it was real, I would be promoting it left, right and centre."

Mr Sampley says he suspected immediately the photograph was a fake, and he decided not to post it on his website. Instead, he claimed, he sent the image to six friends.

"It's a pyramid. I have no idea where it was going or not," he said, adding: "I didn't tell them not to publish it."

Ken Light, the photographer responsible for the original image of Mr Kerry, said yesterday that lawyers for the Corbis agency, which owns the picture, were contemplating legal action.

"It made me feel very angry that it was a complete deception," Mr Light said. "Having one of your images faked in that way for a political cause, or as part of a dirty tricks campaign, doesn't make you very happy."